MOSCOW (Reuters) - Russia is likely to deploy advanced nuclear-capable missiles in its European exclave of Kaliningrad by 2019, casting the move as a reply to a U.S.-backed missile shield, and may one day put them in Crimea too, sources close to its military predict.

That would fuel what is already the worst standoff between Russia and the West since the Cold War and put a swathe of territory in NATO members Poland, Lithuania, Latvia and Estonia in the cross-hairs.

Russia would probably have deployed the missile -- called the Iskander, the Persian name for Alexander the Great -- in Kaliningrad regardless, and the targets it will cover can be struck by longer-range Russian missiles anyway.

But Russian and Western experts say the U.S.-backed shield, which Moscow says is aimed at blunting its own nuclear capabilities, gives the Kremlin the political cover it needs to justify something it was planning all along.

"The Russians plan to do a lot of things they have had in train for some time," said Steven Pifer, former U.S. ambassador to Ukraine, now a senior fellow at the Brookings Institution.

"There's a long history in Moscow of saying what they're doing is in response to what you guys did, even though they planned it in advance."

NATO is holding a summit in Warsaw next month to decide how best to deter Russia after Moscow's lightning annexation of Ukraine's Crimea in 2014. The United States, Britain and Germany have said they will command new battalions in Poland and the Baltics to send Moscow a message.

The summit may prompt Russia to announce counter-measures, but sources close to the Russian military believe Moscow will wait until a planned Polish missile defense site opens in late 2018 to unveil a more serious response.

The Kremlin has often threatened to put nuclear-capable Iskander-M missiles in Kaliningrad, a slice of Russia wedged between Poland and Lithuania, as a riposte to the shield, part of which went online in Romania last month.

But it has kept the West guessing about its real intentions.

'INEVITABLE' DEPLOYMENT

Mikhail Barabanov, a senior research fellow at the Moscow-based Centre for Analysis of Strategies and Technologies (CAST), which advises the Russian Defence Ministry, said it now looked like the Kremlin would deploy them there permanently by 2019.

"By all accounts, the deployment of the Iskanders in Kaliningrad Region is now inevitable," Barabanov told Reuters, saying the missile brigade currently stationed there was using older shorter-range Tochka-U missiles slated for replacement.

The Iskander, a mobile ballistic missile system codenamed SS-26 Stone by NATO, replaced the Soviet Scud missile. Its two guided missiles have a range of up to 500 kilometers (about 300 miles) and can carry either conventional or nuclear warheads.

Russia has twice deployed Iskanders to Kaliningrad on exercises only to reportedly later withdraw them.

U.S. military officials say the U.S.-backed shield which Russia objects to is not aimed at countering a possible Russian threat, but at shooting down missiles from what it describes as rogue states like Iran. Russia says it simply doesn't believe that explanation.

After the United States switched on the Romanian part of the shield, President Vladimir Putin warned Romania and Poland could find themselves targeted by Russian missiles.

"There's a very high chance Iskanders will be deployed in Kaliningrad," Ivan Konovalov, director of the Center for Strategic Trend Studies in Moscow, told Reuters.

"But the Iskanders are our ace card in the standoff over missile defense and NATO's activity around our borders. We need to use it cleverly. There's a big game going on and we don't want to throw it away at the start. We'll play it when Russia needs it most politically."

A marked reduction in East-West tensions might prompt Russia to think twice about the deployment, he said.

Pifer, the former U.S. ambassador, said it was "a matter of time" before the Iskanders showed up in Kaliningrad regardless.

"Kaliningrad is worrisome," he said. "If you have that range of missile there you cover not only all the Baltics but probably about two thirds of Poland."

Russian Defence Minister Sergei Shoigu has said Moscow reserves the right to deploy the Iskanders anywhere in Russia and other senior Russian military officials have said Kaliningrad will get the Iskanders in the next few years as part of a routine nationwide upgrade.

But in the context of many similar threats that have not come to pass, Western experts are unsure if that is a bluff.

Russian experts say it isn't.

"I think the Kremlin will officially drag out the decision (on the Kaliningrad Iskanders) until 2018 or 2019 when the new Polish element of the (U.S.) missile shield will be activated and when the re-arming of other missile brigades throughout Russia with Iskanders is due to finish," said CAST's Barabanov.

CRIMEA MOVE?

The same missiles will probably be deployed in Crimea one day too, he said. Konovalov agreed that was a possibility, but said Russia's Black Sea Fleet was taking delivery of six new submarines armed with cruise missiles and might feel that was enough to counter the Romanian missile site for now.

Some NATO officials privately believe Iskanders may already be in Kaliningrad; others reject that, saying they would have shown up on satellite imagery.

Barabanov said the Iskanders, once deployed to Kaliningrad, would not be armed with nuclear warheads, which are stored in other parts of Russia, but could later be if necessary.

"Deploying nuclear warheads in Kaliningrad would be a separate and serious phase of escalation," he said. Konovalov said arming the missiles with nuclear warheads would be a return to a full-scale Cold War, something he said nobody wanted.

Russian officials have complained that the missile shield launcher systems deployed by the United States in Romania and planned for Poland could be used to fire cruise missiles as well as missile interceptor rockets.

But experts say Moscow, despite its rhetoric, does not yet see the U.S. project as a serious threat.

"Russian missile designers and the military are on record as saying that this system does not pose any threat to our missiles," Pavel Podvig, a Geneva-based researcher with the Russian Nuclear Forces Project, told Reuters.

"Their point is that while we don't necessarily worry about this we do worry where is this all going."

Grigory Podnevolny of online Russian news portal Gosnovosti, a pro-Kremlin site which covers the government, said it would be "scary" when Russia put Iskanders into Kaliningrad and Crimea, but that it was the only way of making America listen.

"It will probably only be when they see how serious the situation is that the Pentagon will want to sit down for talks," he wrote. "The Americans ... always need to stumble into a serious crisis before coming to their senses."